A photometric unit, such as an integrating sphere, is used to determine the total luminous flux or color of a light-emitting device. Luminous flux is the measure of the perceived power of light and can be used as an objective measure of the useful light emitted by a light source. The luminous flux of a light-emitting device can provide an estimate of an apparent amount of light that the light-emitting device will produce.
During testing, a light-emitting device, such as a light-emitting diode (LED), is energized. In the photometric unit, the emitted light from the light-emitting device reflects or diffuses. Diffused light is transported to an optical spectrum analyzer, such as via an optical fiber. After a comparison with data from a test using a reference light under the same testing conditions, the luminous flux of the light-emitting device is obtained.
For example, LEDs can be measured and sorted for color or total luminous flux using integrating sphere-based photometric equipment. The LED is usually placed at a certain distance from a port of the integrating sphere rather than being inserted into the integrating sphere. This results in a significant amount of light loss because some light from the LED does not enter the port and light already inside the integrating sphere escapes through the port. Light collection cones or masks with specular or diffusive reflection surfaces can be used to prevent such light loss, but these devices either still do not allow for complete collection of the LED's light or can significantly alter optical properties of the integrating sphere, both of which result in measurement errors. Furthermore, such errors are difficult to correct through calibrations because the proportion of the lost light varies with an individual LED. The light beam profile or the positioning of the individual LED with respect to the port may not be consistent between LEDs. These variables make it difficult to predict light loss or correct for light loss, such as with a scaling factor. Consequently, these errors contribute to larger tolerances in product specifications, such as with respect to color, lumens, and/or yield loss. Large tolerances in color and lumen specifications can cause LEDs to be ill-suited for particular applications and large tolerances for color or lumen specifications can be undesirable to LED manufacturers.
Figures of merit for such photometric units include accuracy of optical and electrical measurement, throughput, and reliability of mechanical handling. There are tradeoffs among these figures of merit. On some equipment, the LED is kept at a certain distance from a port of the integrating sphere for measurement in favor of higher throughput. Part of the LED's light is then either lost or collected with a device other than the integrating sphere (which may have different optical properties), resulting in various degrees of optical measurement errors. On other equipment, the LED is measured inside the integrating sphere for complete light collection. Throughput is then lowered because the LED has to be longitudinally moved a certain distance (typically from a few mm to a few cm) in order to be put into and taken out of the integrating sphere for each measurement. Reliable electrical contact (required for accurate electrical and optical measurement) and mechanical handling of the LEDs are also challenging at high throughput. Such measurements can involve single LEDs which are only held from the edges (typically only about 0.5 mm wide) by clamps to avoid blocking any light from the LED. The LED is then contacted at its two lead pads (anode and cathode) on its backside by four pogo pin probes which send drive current through the LED to “light it up” for optical testing and measure its forward voltage at the same time. In production testing with adequate throughput, the four probes simultaneously impact the LED with significant mass and speed when they engage, making it difficult for the clampers to reliably hold the LED every time. To secure the LED, an aperture slightly smaller than the top surface of the LED or a window is placed in close proximity above the LED on some equipment, but this results in optical measurement errors because some amount of light from the LED is blocked or reflected. The amount of light that is blocked or reflected varies with individual LEDs.
Two typical tradeoffs are throughput versus complete light collection and throughput versus reliable electrical contacting and mechanical handling. Reduction in LED travel distance typically reduces the amount of light that can be collected. Increases in throughput that involve more contact speed and force from the probes typically reduce reliability. Decreased reliability can result in measurement errors, false rejections, and equipment down time.
LEDs continue to shrink in size as the industry adopts chip scale packaging (CSP). This renders the tradeoff of throughput versus reliability more significant. Smaller LEDs are more difficult to handle reliably and likely limit contact speed of a probe. Smaller LEDs also may require thinner probes, which may be less reliable, and tighter positioning tolerance.
Current photometric units or other test systems suffer from low throughput and are not adequate for manufacturing settings. One current photometric unit has a device flow that is start-stop-start. In an example, an LED is placed proximate an integrating sphere, held motionless for a period while measurements to test color or lumen output are made, and then moved out of the integrating sphere. While the actual test in the integrating sphere may be as short as a millisecond, such a stepped motion may limit overall throughput in the test system to a few LEDs per second.
Therefore, what is needed is an improved test system and, more particularly, a high-throughput photometric unit.